This invention concerns a new method for the formation of abundant quantities of Zinc Silicate from a thermal treatment of a mixture of aluminum oxide zinc oxide and rice or wheat husks in an argon atmosphere.
Zinc silicate (Zn2SiO4) is an important chemical compound with many industrial applications. It is used as an additive to serve as an anticaking agent for selected types of foods; it is widely used as anticorrosion coatings for ships, buildings, and vessels that are exposed to high levels of saltwater. Additionally, in these types of applications, zinc silicate is used as a primer for the paint that is applied to the surfaces.
Zinc silicate, a semiconductor material with a wide band gap of 4.1 eV, is used as a scintillating material in cathode ray tubes and fluorescent electronic devices. Manganese doped Zn2SiO4 is an efficient green emitting phosphor and is extensively used in fluorescent lamps, and plasma display panels [Yang et al., 2005; Morell et al., 1993; Barthou et al., 1994; Rack et al., 1997; Copeland et al., 2002; Thioulouse et al., 1982]. Mn-doped zinc silicate (Zn2SiO4:Mn2+), has been used as a phosphor in fluorescent lamps, neon discharge lamps, oscilloscopes, black-and-white and color televisions, and many other displays and lighting devices for a long time [Harrison et al., 1960; Ronda et al., 1997; Minami et al., 2003; Feldmann et al., 2003; Zhang et al., 2006].
At present, Zn2SiO4:Mn2+ is consumed in high volume for the most advanced televisions; plasma display panels (PDPs), due to its high luminescence efficiency, high color purity, highly chemical and thermal stabilities [Zhang et al., 2006; Liang et al., 2007].
There are two commercial processes for synthesizing zinc silicate. One consists of a solid state reaction at high temperature to produce large pieces of zinc silicate followed by ball milling and grinding to obtain desired powder [Stever et al., 1974]. The other is a wet-chemistry multistep process to produce the powder product [Schulman et al., 1946]. Both processes are time consuming and cost intensive.
Rice and wheat husks are available in abundance as agriculture residues around the world. Rice husk (RH) and wheat husk (WH) have high C and Si contents, so they can be used as a low-cost Si and C sources for synthesizing advanced materials. When WH and RH are burned at a certain temperature in air, carbon and other elements are released leaving silicon dioxide (SiO2) as the major component of the end product known as rice husk ash (RHA) or wheat husk ash (WHA). Recently Xiong et al. showed using a multistep process, that SiO2 extracted from RHA can be utilized and mixed with Zinc Oxide (ZnO) to produce Zn2SiO4 and when doped with Mn2+ results in an efficient phosphor material. The following equation describes the solid state reaction that will result in zinc silicate:2ZnO SiO2=>Zn2SiO4 In addition to phosphor characteristics of zinc silicate, it has been shown that nano-rod like structures of Zn2SiO4 are capable of adsorbing heavy metals such as Fe, Cd, Hg and Pb in water [Wang et al., 2012]. With these findings, there is added incentive to study the properties and performance of Zn2SiO4 and its applicability of purifying water by removing heavy metals.
Previous work on agriculture residues has shown successful conversion of rice husks, wheat husks, sorghum leaves, and corn leaves into silicon carbide (SiC) and Si3N4 by pyrolyzing in argon atmosphere or in nitrogen atmospheres respectively [Gorzkowski et al., 2013; Qadri et al., 2012; Qadri et al., 2013; Qadri et al., 2013; Qadri et al., 2015; Qadri et al., 2013; Qadri et al., 2015].
Importantly, this type of synthesis involves a single step process in which no additives are involved. This disclosure focuses on producing nanoparticles of Zn2SiO4 from wheat and rice husks by adding ZnO in the correct weight ratio with SiO2 that is inherently present in these agriculture residues.
This process does not produce RHA or WHA, but rather uses the natural content of SiO2 present in RH or WH.
The carbon content acts as catalyst in the solid state reaction and the remaining carbon present in the sample can be removed by heating at an appropriate temperature in air. The structural parameters and composition of the produced Zn2SiO4 are investigated using x-ray diffraction (XRD) and Raman scattering spectroscopy. In addition, transmission electron microscopy results are presented to obtain the size and shapes of the nanocrystals of zinc silicates.
The phosphor characteristics were determined in-situ by using x-rays of 8 keV coupled with an optical spectrometer, and room temperature photoluminescence (PL) and PL imaging.
Billions of pounds of agricultural waste, such as rice and wheat husks, are generated every year all over the world. Zinc Silicate is a very useful material for industrial applications due to its unique physical properties. Examples of uses include but are not limited to an additive as an anticaking agent for selected types of food, excellent anticorrosion coatings for ships, buildings, and vessels exposed to high levels of salt water, a primer for the paint that is applied to the surfaces, a fluorescent and phosphorescent material for wavelength between x-rays and UV light and as a scintillating material in cathode ray tubes and fluorescent electronic devices.
Here, Zinc Silicate is synthesized from wheat and rice husks or other husks using conventional heating or microwave heating to produce nano-structures previously unseen.